In the fabrication of semiconductor memory devices it is common for an array of memory cells to include one or more defects which prevent the proper performance of the memory circuit. If a type of defect occurs systematically it can often be causally analyzed and designed out. Other defects which are generally not systematic include short circuits between adjacent columns and open circuits within individual columns of memory cells. For analysis purposes the distribution of such defects in a memory device, as well as the distribution of the number of defects among a given production lot, may be considered random so that the yield of good devices in a lot can be modelled according to a Poisson distribution function. Typically, over the period of time that a particular device or family of devices is being produced in a given manufacturing facility the product yield can be improved by removing causes, e.g., particulate matter, of the above-mentioned random defects.
Unfortunately, the causes of random defects cannot be completely eliminated in many fabrication processes. Redundancy schemes have been introduced to reduce the effect these defects have on device yield. The redundancy schemes normally consist of a few extra rows and columns of memory cells that are placed within the memory array to replace defective rows and columns of memory cells. In theory, these extra rows and columns of memory cells should maximize the yield of a production lot. Practically, however, many potentially functional devices must be rejected because the number of available redundant elements on the chip prove to be insufficient to meet the repair needs of the device.
One possible solution to the problem of defects is simply to increase the number of available redundant elements. Area constraints of dense modern memory devices, however, determine an upper limit to these types of additions. What is needed is a novel approach of incorporating an additional level of redundancy that can be used to obtain a functional product once the standard redundancy system has been exhausted.